Method and apparatus for well logging



W. L- RUSSELL METHOD AND APPARATUS FOR WELL LOGGING Oct. 3, 1950 FiledDec. 15, 1946 humane" AMPLIFIER -33 ,3I- L-/\ mp 34 W AMPLIFIER MOTORHORIZONTAL COMPONENT ECTION DIP ANGLE TOTAL FIELD 3 Sheets-Sheet 1 4 6 2GYROSCOPIC Z STABILIZER 1| 22 MOTOR 4 4 so s? I 82 2 8| i 2 as' g: 4 w63 I 9 .64 3 Z I 86 6 62 s1 s7-- 6 as 8 3 2 INVENTOR.

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METHOD AND APPARATUS FOR WELLv LOGGING Filed Dec. 15; 1946 3Sheets-Sheet 2 To Reta rda r To Recorder faaauzucv DOUBLER Paiente st.3, 1950 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR WELLLOGGING William L. Russell, College Station, Tex. Application December13, 1946, Serial No. 715,915 17 Claims. (01. 175-182) This inventionrelates generally to the logging of wells and is directed particularlyto the logging of certain magnetic properties of well formations bywhich correlations may be carried out over large distances independentof the occurrence of marker fossils or lithologic similarities. The dataso obtained are also of considerable value in improving magneticcore-orientation methods.

For correlation over short distances electrical,

radioactivity, and drill-cutting or core-sample logs have proved veryuseful and reliable. However, since they are dependent on lithologicvariations, they may be used only over those distances where thelithologic succession is persistent. These distances may vary from a fewhundred feet to hundreds of miles.

For correlation over larger distances and where the lithology is notpersistent, fossils have heretofore been relied on with considerablesuccess. However, because the volume of material brought up during thedrilling of wells is comparatively limited, fossils easily found atsurface exposures may by chance be ,absent from the particularsubsurface samples obtained or may be so damaged by the drill as to beunrecognizable. Further and varying factors such as, for example,changes in environment or separation ofthe areas of deposition bybarriers often so complicate the fossil-correlation method as to make itimpractical or impossible to apply. Also, large areas where fossilcorrelation would be desirable are covered to great depths by red bedsand other strata of non-marine origin which are nearly always devoid offossils suitable for correlation.

In the orientation of cores by measuring their weak remanent magnetism,the accuracy of the results is dependent on the precision with which thedirection of the remanent field in the particular stratum is known orcan be ascertained. It is often necessary to assume a remanent fielddirection parallel to the present earths induced field. However, fromobservations at outcrops and other information, it is well known thatremanent field directions vary, so that any assumption that they areconstant or the same is bound to lead to serious errors upon occasion.

It is accordingly a primary object of my invention to provide a methodand apparatus for making well logs correlatable over great distancesregardless of lateral changes in lithology. Another object is to providea method and apparatus for logging certain magnetic properties of wellformations, which properties are related to the earth's magnetic held atthe time of deposition of each stratum rather than to the nature of thematerials forming the stratum. A fur-.

ther object is to provide a method and apparatus for making well logsthat are correlatable on the basis of the time of deposition of thevarious strata rather than the nature of the minerals in the strata, sothat strata of the same age in different locations may be identified assuch even though their mineral compositions differ. Still another objectis to provide a method and apparatus for obtaining data from which thenature of the variations in the earth's magnetism in past geologicalages may be determined. A still further object is to provide a methodand apparatus for obtaining remanent field direction data for thevarious well strata, which data are directly applicable to improving theinterpretation of magnetic core-orientation measurements. Many other andfurther objects, uses, and advantages of my invention will becomeapparent as the description proceeds.

In general terms the essence of my invention by which the foregoing andother objects are accomplished comprises the logging or measurement inwells of the variations in the orientation and inclination of certainresidual or remanent magnetic properties.

The magnetic field encountered in wells may be considered as arisingfrom two sources: the flux induced in the strata by the'earths presentmagnetic poles, and the remanent field. The former is generally themajor component of the resultant field, while the remanent magnetism isusually quite small by comparison. ,Whereas the variations in thestrength of both the induced and the remanent magnetic fields are oftenconsiderable, they are generally related to the kind and composition ofeach rock, beir'fg dependent on theeontent of' magnetite. To bedistinguished from these variations, on the other hand, are thevariations in direction of the remanent field which are related to thechanges in direction of the eart s magnetic field during the time ofdeposition These latter variations result from the movements of themagnetic poles which both have of the sedimentary rocks.

persist- The remanent magnetism of rocks is apparently produced almostentirely by the mineral magnetite. Since magnetite rarely forms inunmetamorphosed sediments, the magnetite now in them was probablyderived from igneous and metamorphic rocks. This limits its occurrenceto clastic strata. As the elastic particles containing some magnetitesettle through the water or air from which they are deposited, themagnetite tends to become oriented so thatitsmagnetic field is parallelto the earths field. As the particles come to rest on the surfacebeneath the fluid, they tend-to retain the same orientation, and oncethey have been buried by later deposits theyv are unable to move inresponse to subsequent changes in the orientation and dip of the earthsfield.

The magnetic permeability of rocks appears also to be due to themagnetite content, and like the process of orientation of a particl byits remanent field, a certain amount of orientation of the particlesoccurs due to this property. In other words, in so far as a particle hasa greater permeability in one direction than inany other direction, thiswill be the direction in which the particle becomes magnetized whileexposed to the earths magnetic field during settling to the bottom ofthe fluid. by later deposits thus prevents further movement to align themaximum permeability axis with the shifting direction of the earthsfield in the same way that the remanent magnetism becomes fixed. Thereare thus two properties-the direction of the remanent magnetism and thedirection of the axis of maximum permeabilityavailable for the purposesof this invention.

All rocks deposited from solution would be devoid of magnetite if pure,so that their magnetic properties could not be logged. However, manylimestones, dolomites, and other rocks deposited primarily from solutionare found to contain in their elastic impurities sufflcient magnetitefor logging. Moreover 80 per cent or more of all sedimentary rocks areelastic.

Whereas it is a relatively simple matter to measure the directionalmagnetic properties of small rock samples in the laboratory, themeasurement of these properties in wells is tremendously complicated bythe presence of the earths field. Consequently the magnetic fieldobserved by the well-logging instrument, while it involves rock massesmany times. the size of laboratory samples, is the resultant of the twofields: the induced field and the remanent field of which it is desiredto measure the orientation and dip. While it is quite difiicult orimpossible to eliminate completely the efiect of the induced field onthe measurements, it can be evaluated with sufficient accuracy for thepurposes of this invention.

Since the induced field is generally many times larger than the remanentfield, for the purpose of estimating the magnetite content of rocks, theremanent field strength may be neglected and the resultant and inducedfields considered equal. It is chiefly their magnetite content whichdetermines the permeability of rocks and gives rise to the variations inthe induced flux. As the strength of the remanent field is alsoapproximately proportional to the magnetite content to which it is due,this same measurement of the resultant field strength is useful for thepurpose of estimating the remanent fieldstrength. Therefore, in thepractice of my invention the strength of the resultant field in thevarious strata is logged along with the variations in its direction, orthe Fixing of the particle in place variations in direction of themaximum permeability, or both. By properly employing the data thusobtained logs related to the direction of the remanent field, or to thedirection of the maximum permeability, which is generally the same, maybe computed. These are the logs useful for both long and short distancecorrelations, and from which information may be derived as to thevariations in the earth's magnetism in past geological ages.

These principles and their relation to my invention will be betterunderstood by reference to the accompanying drawings forming a part ofthis application for purposes of illustration. In these drawings thesame reference numeral is applied to the same or a corresponding part inthe different figures. Thus,

Figure l is a cross section of a well with an instrument embodying theinvention shown generally therein;

Figure 2 is a cross section of one embodiment of the detectors of myinvention;

Figure 3 is a cross section of Figure 2 on the lines 3-3;

Figure 4 is a. cross section of an alternative embodiment of thedetector units of my invention;

Figure 5 is a circuit diagram applicable to the detector embodiment ofFigure 4;

Figure 6 is a circuit diagram of the fieldstrength detector of Figure 4;

Figure '7 is a cross section of a third embodiment of my inventionadapted to measuring the anisotropy of permeability around a well bore;

Figure 8 is a cross section of Figure '7 on the lines 88;

Figure 9 is a circuit diagram of the apparatus of Figures 7 and 8showing the method of recording the data; and

Figure 10 is a graphical representation of an assumed permeabilitydistribution and the resultant detector signal.

In Figure 1 is illustrated a general arrangement of the apparatus formaking measurements of the various directional properties of the earthsmagnetism in accordance with my invention. Adapted to be lowered on acable 10 into a well I I is a suburface instrument llhaving a rigidouter housing l3 of non-magnetic material strong enough to withstand thehydrostatic pressures encountered in wells. Centralizing springs l4 andI5 at either end of housing [3 may be provided to hold the instrument I!in the center of the well when it is necessary to do so.

In suitable spaces within housing 13 are locate power-supply equipmentI6, which may include batteries or alternating-current sources or both,depending on the specific types of detectors employed, and amplificationequipment i'I consisting of one or more separate amplifier channels, asneeded, for signal transmission and the like. Mounted in a frame l8,which is preferably freely suspended from the center of a cross memberis by a rotatable and flexible connection 20 so as to hang vertically ina compartment near the lower end of housing 13, is the magnetic-field orpermeability-direction measuring equipment. When and if housing l3 tendsto rotate, a gyroscopic stabilizer 2| mounted in'frame I8 keeps itoriented in a fixed predetermined direction. Where themagnetism-measuring devices are such as to require mechanical power fortheir operation, it is supplied by an electric motor 22 likewise fixedin frame l8, it being understood that precautions as by shielding orotherwise (not shown) will be taken to the magnetic fields and materialsof the motor and gyro and the earth's directional magnetic propertiesbeing measured.

The apparatus carried by frame l8 for these measurements includes atleast one, and preferabl two or more detectors sensitive to thedirections or direction changes of components of the remanent magneticproperty in different planes. Thus, a detector 23 is primarily sensitiveto changes in direction in a horizontal plane of the horizontalcomponent of the measured property, while the detector 24 is sensitiveto changes in its dip, or more generally to changes in direction of thecomponent lying in a given vertical plane. A detector 25 is responsiveto changes in the total magnetic-field strength or induction in the wellformations so that the effect of these changes on the directionmeasurements can be allowed for.

After amplification of the responses of detectors 23, 28, and 25, asnecessary, by suitable channels in amplifier II, the resulting threesignals may be transmitted to the top of well I I over separateinsulated leads in cable It. Such further amplification as is necessaryfor recording is provided by a horizontal-direction signal amplifier 30,a dip signal amplifier 8|, and a field-strength signal amplifier 32,each of which amplifiers drives one of the respective recording devices38, 34, 85, and thereby records on a moving chart 86. Movement of chart88 in accordance with the depth of instrument l2 in the well isaccomplished by any of the conventional expedients of well logging, suchas by a driving connection 31 actuated from a depth-measuring sheave 88over which cable Ill passes. Since the signal transmission and recordingin correlation with instrument depth may follow any of several suitableand conventional practices, no further detailed description of them isbelieved necessary herein. Such modifications of these partsof thesystem as may be required to adapt them to the various specificembodiments of the detection equipment described hereinafter will beapparent to those skilled in the art of well logging.

One such embodiment of the measuring equipment is shown in some detailin Figures 2 and 3, where it will be seen that the suspension 20 forframe [8 includes an upper 48 and a lower tubular member ll connected bya flexible or universal joint 42, upper tubular member 40 being heldinflexibly but free to rotate by a bearing 43 in cross-member I8.Electrical conductors 48 supplying power to or carrying signal currentsfrom the apparatus mounted in frame l8 extend through tubes 48 and lland are brought out to slip rings 45 mounted on tube 48 and contacted bybrushes 6. It is to be understood that the number of these slip ringsand brushes will vary depending on the number of independentelectrical-circuit connections to be made between the fixed and thesuspended equipment in hous- 1118 I8.

Best shown in Figure 2 is the dip-detector assembly :4 which in thisembodiment is adapted for measuring the angle of inclination of themagnetic field intersecting the well bore at various depths. Theassembly consists of a pair of highly V permeable ferromagnetic bars 68and II placed end to end and having between their adjacent ends an airgap in which is located a rotatable inductor coil 52. This coil ismounted for rotation about its diameter on a shaft I8 journaled in apair of non-magnetic bearing members 84 and minimize interferem between6 56, which may also help to support the bars 88 and BI as a rigid unit.The outer ends of the bars 50, 8! are adjustably clamped to frame 18 bythe clamps 56 and 51 movable in slots in the frame l8. The winding ofcoil 52 may have one terminal grounded and the other brought out to aninsulated slip ring 58 contacted by a brush 58 connected to one of theleads 44.

The horizontal-component direction detector 28 is generally similar tothe dip detector 24, as can be seen by referring to Figure 3. Detector23, likedip detector 28, consists of a pair of highly permeableferromagnetic bar members it and 8| aligned end to end with an inductorcoil 62 in an air gap between them. Non-magnetic bearing members 83 andcarry a, shaft 65 on which the inductor coil 82 is mounted for rotationabout its diameter and also support the bars 80 and 8| as a unit, theirouter ends being adjustably fixed to the frame l8 by clamps 66 and 61.One terminal of the coil 62 is grounded, and the other is brought out 88connected to another one of the leads II.

The field-strength detector 25 may be simply an inductor coil 18(Figurer2) fixed on a shaft H for rotation about its diameter, one endof the coil winding being grounded and the other brought out to the slipring 12 contacted by the brush 13 connected to a still dlfierent one ofthe leads H. Shaft II rotates in bearings contained in a gear box 14 andan adjustable support member 15, both of which are clamped in slots inthe frame l8. In order to generate voltages indicative of the magneticfields surrounding them, the inductor coils 52, 82, and are rotated bysuitably coupling them to the motor 22, which is preferably of aconstant-speed type. While this coupling is a positive one in order topreserve the phase relations between the various coils, it is alsoflexible so as to allow some adjustment of the measuring .units in theframe l8. Thus, the motor shaft is connected by universal joints 80 and8| and a spline 82 to a gear box 83 adjustably clamped in a slot inframe l8 by a clamp 84.

Thence torque is transmitted by a similar universal joint and splinecoupling 85 to the shaft 65 of the inductor coil 82. Another flexiblecoupling 88 simultaneously transmits power from 60 the box 83 to a,second gear box 81 held by the clamp 88. From here the shaft63 of the inductor 52 is driven by a coupling 89. A similar coupling 88 from shaft58 to the gear box 14 pro vides for rotation of the inductor coil 10. Itis 58 thus seen that all of the inductor coils are linked together so asto be driven in unison by the motor 22, but the flexibility'of thecouplings permits limited movement of the various units in the frame I8.60 The adjustment and operation of this embodiment of my invention mayfollow either of two general procedures. It is assumed that the magneticfield observed at the earth's surface at the well location representsthe normal values of .08 both the direction and intensity in that arearelatively unaffected by the remanent fields and the varyingpermeabilities 'of the strata below. It is also assumed, from the knownfact that the contributions of the '-remanent fields to the ob- 10served total field strengths are small, that the .varlations indirection of the resultant fields in the strata will not exceedrelativeiy narrow limits. Further, it is obvious that when the axes ofthe bars 80, 8i and 80, II are perpendicular to 76 a magnetic field, theinduction in each is equal,

to a slip ring 68 contacted by a brush 7 and no flux crosses the gapsbetween them to be intercepted by the inductor 52 or 62 during rotation.

One adjustment procedure therefore consists in so adjusting and settingthe clamps and bars that the axis of bars 60, CI is horizontal andperpendicular to one of the expected limiting direc tions of themagnetic field horizontal component to be encountered in the well. Bars50. are so oriented and clamped as to lie in the vertical plane whichvincludes the surface direction of the magnetic field, their axis beingturned in this plane so as to be perpendicular to one of the limits ofvariation in magnetic dip to be expected. The axis of rotation, i. e.,shaft 1|, of inductor II is placed as nearly as possible perpendicularto the direction of the surface magnetic field. It may or may not lie inthe same vertical plane as bars I, II. The axes of rotation of the smallinductors 52 and i2, 1. e., the shafts 53 and 65, are orientedapproximately parallel to the surface field direction so thatsubstantially the only flux intercepted by them during rotation will bethat crossing the gap from one bar to another.

well being the significant indications. The in- As the amplitude is, asbefore, a function of both the strength and the direction of theresultant field in a stratum, the indications may be corrected in amanner similar to the first operating procedure: either by applying thecorrection factor previously mentioned or byrecording the quotients ofthe outputs of each of the directional detectors by the output of thefieldstrength detector. The polarity of the variations may convenientlybe determined by using the signal from the inductor II as a referencestrument is then passed through the well, and

the amplitudes of the three signals are recorded as functions of depthon the chart 38.

It is believed apparent that in this case the amplitudes of the twodirectional detectors 2! and 2i will vary with both field strength anddirection. Neglecting variations in strength for a moment, the outputswill be zero for the limiting field directions that are perpendicular tothe bar axes, of medium amplitudes for directions parallel to thesurface field, and of maximum amplitudes for field directionsapproaching the other assumed limit of directional variation. Theinfluence of variations in field strength is eliminated from the finallogs, however, by applying to the uncorrected values a correction factorwhich is the ratio of the total field strength at the surface to itsvalue at the stratum in question in the well. This is the ratio of thesignal amplitude of the inductor III at the surface to its value in thewell, and may be employed either in the subsequent preparation ofcorrected logs, or at the time of the logging operations. In the lattercase recorders 33 and 34 may be adapted to record, not the signalamplitudes directly, but their respective quotients with the total fieldstrengths for the corresponding strata.

In the second and ordinarily preferred adjustment procedure, the baraxes are oriented perpendicular to the surface directions of thecomponents of the magnetic field to which they are responsive, insteadof perpendicular to .one of the limiting directions expected in thewell. Consequently the outputs of the inductors 52 and 62 are zerowhenever the field component direction in the well is the same as it isat the surface, and the direction of a variation from the surfacedirection is easily ascertained from the fact that thealternating-current output of 'the inductors 52 and 62 shifts 180degrees in phase as the direction changes from one side of the surfacereference direction to the other.

cated at an apex of the triangle.

with which to compare the phases of the signals from the inductors 52and $2, for the reason that the phase variations in the field-strengthsignal are small because the changes in total field direction are small.The manner of applying a phasemeter to this determination will beapparent to those skilled in the art.

In Figure 4 is shown an alternative embodiment of my invention which issimpler mechanically than the embodiment of Figures 2 and 3' in that norapidly moving parts are employed corresponding to the various rotatinginductor coils. Briefly, these detectors comprise stationaryferromagnetic cores exposed to the steady magnetic field andperiodically saturated by additional pulsating or alternating fluxesinduced therein by exciting windings. Pickup coils about the cores areso arranged that when there is no external steady field the voltagesinduced in them by the additional fluxes balance out. But in thepresence of such a field the balance is disturbed, giving a resultantvoltage which may be used to indicate the field strength or itsdirection relative to the cores, or both.

As, in .this embodiment, the horizontal-component detector 23 and thedip detector 24 may be practically identical except for theirorientation in the frame. ll, only the latter will be described indetail, since it is shown to better advantage in the drawing. Forclarity the electrical leads and connections have been omitted here.

Arranged as the sides of a triangle, preferably equilateral, are thethree highly permeable, saturable ferromagnetic cores Ill, Ill, "2,carrying the respective exciting windings I, Ill, Ill and the respectivesets of pickup windings III, I01; I", I09; and III, III. Althoughtheyare not strictly essential, the induction in the cores due to anexternal field is increased by the armate pole pieces II2, III, Ill,each of which is 10- These pole pieces may conveniently carry the clampsIII, II by which the unit 24 is fastened in the supporting frame I8.

The field-strength detector 2' in this embodiment may consist of asingle permeable, saturable ferromagnetic core I20 having arcuate polepieces HI and I22, the tips of which are bent backward towards thecentral part of core I2. so as to formnearly closed fiux paths with thetwo halves of the core. Within each fiux path and surrounding core I20are the exciting-windings I23 and I24, while a single pickup winding I2lencircles core I20 at its center. Mounting of the entire assembly on adisc or plate I2, pivoted to a cross-member I2'I clamped to frame II.provides for adjusting the direction of the core axis, plate I2 beingfixed in a desired position by a screw I28 clamping it to cross-memberI2'I.

The circuit connections of this embodiment of my invention are shown inFigure 5. Although the exciting coils I. III. III are there shownconnected in series and to a suitable source I" of alternating current,they can equally well be in parallel rather than series. The pickupcoils are Y-connected in pairs, coils I06 and III forming one pair, I01and I06 another, and I09 and H a third. As may be observed from aninspection ofthe drawing the relative directions of these pickup andexciting windings are such that the voltages induced in each coil of apickup pair by the alternating exciting fluxes tend to balance out; forexample, the voltage in coil III due to the flux excited by coil I03 inits core is equal and opposite to the voltage in coil I06 due toexcitation by coil I04, and so on for the other two pairs. However, aresultant voltage appears in each coil pair, considered as a unit, ifthe induction due to an external steady field (which depends both on thefield strength and its direction relative to the core) brings one corecloser to saturation than another, so that the induced voltage isdistorted by a saturation "cut-off" effect in one coil different fromthat in the other coil of the pair.

In the presence of such an external field, each of the three coil pairswill generally show such a resultant voltage, ach proportional to thecoma ponent of the field in one of the three-core directions. The vectorcombination or resultant of these three individual voltages is then thedirection and strength of the total field component lying in the planeof the triangular core structure. For transmitting this information tothe appropriate recorder (either the horizontal recorder 33 or the diprecorder 34, depending on the orientation of the triangular detectorunit in frame I6) the outer terminals of the Y-connected coil pairs maybe connected by the three leads I3I to a 3-coil Y-connected stator I32consisting of the coils I33, I34, and I35 oriented 120 apart. As thesethree voltages and their resultant are of twice the frequency of sourceI 30, the direction of the resultant in stator I32 is indicated by theposition of a rotor I36, to which is supplied a current of the samedouble frequency and proper phase from the source I30 through afrequency-doubling circuit I31. The alignment of rotor I36 is anindication of the resultant field direction, and for transmission to therecorder,

for example, recorder 34, it may be made to adjust the contactor I36 ofa potentiometer I 39 in series with a battery I40, thereby sending tothe recorder a direct-current signal over the leads l4l indicative ofthe magnetic-field dip in the well.

As the magnitude of the resultant voltage in stator I32 is proportionalto the total strength of the magnetic field, in the case of thisembodiment of the dip detector 24, it may be measured there in place ofusing the separate detector 25. This may be accomplished, for example,by a suitable pickup coil having its axis oriented by the rotor I36 to aposition perpendicular to the resultant field of stator I32.

However, as shown by Figure 6, the exciting coils I23 and I24 of thepreferred separate fieldstrength detector 25 of this embodiment areconnected in series opposition and to the source I30. When core I isaligned parallel to the average direction of the magnetic field or toits direction at the surface, then the double-frequency unbalancevoltage in pickup coil I varies in amplitude directly with the fieldstrength. If desired, a rectifier I42 may convert this to adirect-current voltage for recording by the recorder 35.

Source I may also supply the exciting voltage for thehorizontal-component detector 23 which is exactly likethe detector 24.However, a separate rotor and stator unit is required to analyze thethree components of the horizontal flux and transmit an indication oftheir resultant to the direction recorder 33.

In operation, it is only necessary at the surface to set the detector 23in a horizontal plane, the

detector 24 in the vertical plane which includes the surface fielddirection, and the field-strength detector core I20 parallel to thesurface field. Thereafter the gyro 2| maintains the orientation of.frame I8 while the instrument is passed through a well. If desired, thedirectional logs may be corrected by the output of the detector 25during the logging run as in the Figure 2 embodiment, or the data fromthe three detectors may be recorded directly for use in preparing corvmeasure other components of the permeability than its values in a planeperpendicular to the well axis; that is, the horizontal component in avertical well.

The apparatus for this measurement consists of a central tube I on theaxis of the oriented frame I8 adapted to be rotated by the motor 22through gears I5I and I52. A cross arm I53 is clamped at its center totube I50 and carries at its extremities the coils I54 and I55, the axesof which lie along cross arm I53. Fastened also to cross arm I53 at itscenter are the permanent bar magnets I56 and I51 with their axes alsoaligned parallel to the arm. Magnetic shields I58 and I53 above andbelow these magnets and coils isolate this section of the apparatus fromthe remainder of the instrument. Although they are not essential, it isoften 'helpful to balance out the earth's magnetic field at leastpartially by a plurality of small permanent magnets I60 set in the outerwall of the portion of frame I8 included between shields I58 and I59.The coils I54 and I are connected electrically in series and their leadsare brought out through the interior of tube I 50 to slip rings I62contacted by brushes I63, whence suitable leads extend to an amplifierand the recording system.

Also mounted on the tube I 50 in a position to be shielded from themagnets I56, I51, and I is an inductor coil I65 rotatable by tube I50about the coil diameter. The leads for this coil I65 may also be broughtout through the tube I 50, slip rings I62 and brushes I63.

The tooth ratio of gear I5I to I52 is preferably 1:2 so that the shaftof motor 22 rotates at exactly twice the speed of tube I50. Coupleddirectly to motor 22 is a small single-phase alternator I66 whichtherefore generates a current of twice the frequency of rotation of tubeI50.

The circuit connections for this embodiment of my invention are shown inFigure 9. The coils I54 and I55 are connected to a filter I10 whichpreferably passes only a frequency equal to twice the frequency ofrotation of shaft or tube I50, and in particular rejects that frequencyequal to the rotation speed. The signal from this filter is thenamplified by an amplifier HI and passed vention to one set of inputterminals on a phase-meter or bridge I12. The other input to phasemeterI12 is taken from a double-pole double-throw switch I15 which connectseither to the inductor IE or the alternator I66. The output of inductorI65 is preferably fed through a frequency doubler I10. The signal fromphasemeter I12 may be transmitted directly to the recorder for recordingin correlation with depth, the log then being simply a trace varyingaccording to the phase angle between the two voltages entering meterI12.

The operation of this embodiment of the incan best be understood fromFigures 8 and 10. Let it be assumed that the permeability is a maximumin the direction of the arrows I18 and a minimum in the direction atright angles thereto designated by the arrows I11. Then, as the coilsI54 and I55 rotate, along with magnets I58 and I51, some of the fiuxfrom the latter will pass through the coils and enter the formations ofwell II. Because of the greater permeability in the direction I16, moreflux will pass through the coils when cross arm I53 points in thisdirection and less when it points at right angles, paral- 101 to thearrows I11. These positions of .maximum and minimum flux are reachedtwice for each revolution of the shaft I50, so that the induced voltagedue to the permeability effect will have twice the frequency ofrotation. On this basis it can be differentiated from the signal due tothe rotation of coils I54 and I55 in the unbalanced portion of theearth's magnetic field, which will be the same as the frequency ofrotation.

The graphs of Figure 10 will show this even more clearly. Adopting thearrow I18 (Figure 8) as the reference direction, the upper graph showsthe assumed variation in permeability around the well II. It varies froma maximum value of a: at about 1r/4 to a minimum of in at 31r/4, back toa maximum as at 51r/4 and to a minimum n again at hr/4. As the voltagein coils I55 and I55 is proportional to the time rate of change of fiuxthrough them passing into the formations from magnets I56 and I51, it isrepresented by the lower graph, which, it will be observed, is displacedby a phase angle of 1r/4 from the permeability graph and has a frequencyof twice the frequency of rotation of the coil-magnet structure.

As the direction of the horizontal component of the magnetic field doesnot vary greatly because it is largely due to induction rather thanremanent magnetism, it is used as the reference direction when switchI13 is thrown to the upper position. Alternatively, when switch I13 isin the lower position the output of alternator I66, which has adirectional sense because of the orientation of frame I8 by the gyro 2|,is used as a reference signal. Either one is adequate to indicate thevariations of the direction of anisotropy of the permeability in thedifferent strata of a well.

The logs made in the practice of my invention are useful in many ways,for example, in direct correlation from well to well by the similaritiesof appearance of the logs, or for the preparation of master logs showingthe attitude of the earth's magnetic field at the various times ofdeposition of the strata. Also, the data will be found highly useful inimproving the accuracy of core-orienting procedures which rely on theremanent magnetism of cores for directional orientation.

Although in the case of the remanent magnetism it has been proposed tomeasure more than one component, i. e., the horizontal component and thedip, it will be appreciated that most of the advantages of the inventionare retained ,if logs are made of only one component, such as thedirection of the horizontal component.

While the invention has been described in terms of the foregoingspecific embodiments thereof, it is obvious that many modifications willoccur to those skilled in the well-logging art. The invention shouldtherefore not be considered as limited solely to these specificembodiments but is best defined by the following claims.

I claim:

1. The method of logging wells which comprises the steps of measuring ata plurality of depths within a well the direction of the horizontalcomponent of the earths magnetism, said magnetism including fiux'due toboth the induced and the remanent fields, measuring at a plurality ofdepths the dip of said magnetism, and recording separately thevariations ofsaid direction and of said dip as functions of depth.

2. The method of logging wells which comprises the steps of producing afirst electrical signal varying with the direction of the horizontalcomponent of the magnetic field in a well, said field being theresultant of both the induced and the remanent magnetism, producing asecond electrical signal varying with the dip of said magnetic field,producing a third electrical signal varying with the strength of saidfield, and recording the variations in each of said signals as functionsof depth.

3. The method of logging wells which com-' netic field in the well, andrecording the varia-v tions of each of said signals as functions ofdepth.

4. The method of logging wells which comprises the steps of, at theearth's surface, orienting a first elongated ferromagnetic coreperpendicular to the earth's magnetic field and in a horizontal plane,orienting a second elongated ferromagnetic core perpendicular to theearths magnetic field and in a vertical plane, passing said coresthrough a well while maintaining their orientations, and recording asfunctions of depth in said well the variations in fiux traversing saidcores longitudinally.

5. The method of logging wells which comprises the steps of, at theearth's surface, orienting a first elongated ferromagnetic core in ahorizontal plane and perpendicular to one of the limiting directionsexpected of the varying horizontal component of the magnetic field in awell, orienting a second elongated ferromagnetic core in a verticalplane and perpendicular to one of the limiting directions expected ofthe varying dip of the magnetic field in said well, passing said coresthrough said well while maintaining their orientations, measuring thestrength of the magnetic field in said well, and recording as functionsof depth in said well said magnetic-field strength and the variations inmagnetic flux traversing each of said cores longitudinally.

6. The method of logging wells whichcomprises the steps of exposing tothe magnetic field of the formations of a well a plurality of saturablerromagnetic cores arranged as the sides of a l3 polygon forming a planeof investigation, inducing additional periodic saturating fluxes in saidcores, measuring the differences in said saturating fluxes in said coresdue to the presence of fiux from said formations therein, determiningthe resultant direction of said differences, measuring the totalintensity of the magnetic field in said well formations, and recordingas functions of depth in said well the variations of said resultantdirection and said total intensity.

7. The method of logging wells which comprises the steps of impressingon the formations of a well at varying depths therein a constant fluxwhile rotating the source of said flux about the axis of said well,intercepting a portion of said fiux passing into said formations,producing an electrical signal varying with the amount of interceptedfiux around said well, and recording in correlation with depth thevariations in phase of that component of said signal having a frequencyequal to twice the frequency of rotation of said source.

8. The method of logging wells which comprises the steps of impressingon the formations of a well at varying depths therein a constant fiuxwhile rotating the source of said fiux about the axis of said well,intercepting a portion of said fiux passing into said formations,producing an alternating electrical signal proportional to thevariations in said intercepted flux around said well, said signal havinga frequency equal to twice the frequency of rotation of said source,producing a second electrical signal by rotation of an inductor in theearth's magnetic field, doubling the frequency of said second signal,and recording the variation in phase between said alternating signal andsaid doubled second signal as a function of depth in said well.

9. The method of preparing well logs which comprises the steps ofmeasuring as a function of depth in a well the direction of 'a componentof the magnetic field therein, which measurement also varies with thestrength of said field, measuring as a function of depth the strength ofsaid field, and plotting in correlation with depth a quantityproportional to the quotient of the direction measurement and thefield-strength measurement.

10. The method of preparing well logs which comprises the steps ofmeasuring as functions of depth in a well the directions of a pluralityof components of the magnetic field therein, which measurements alsovary with the strength of said field, measuring as a function of depththe strength of said field, and plotting in correlation with depth aquantity varying with the quotient of each component directionmeasurement and the field-strength measurement at the correspondingdepth.

11. Apparatus for logging wells comprising an instrument housing adaptedfor lowering intoa well, a reference framework in said housing andmovable relative thereto, means for maintaining said framework orientedin a fixed direction, means carried by said a first electrical signalvarying quantitatively with the direction of a directional magneticproperty of well strata, means carried by said framework for producing asecond electrical signal varying quantitatively with the strength of themagnetic field in the well strata, and means for recording thevariations of each of said signals in correlation with depth in a well.

12. Apparatus for logging wells comprising an instrument housing adaptedfor lowering into a framework for producing aoa aoo well, a referenceframework in said housing and movable relative thereto, means formaintaining said framework oriented in a fixed direction, at least onemagnetic induction means carried by said framework and capable ofvarying an elecinstrument housing adapted for lowering into a well, areference framework in said housing and movable relative thereto, meansfor main said framework oriented in means for producing a firstelectrical signal varying with the direction of the horizontal componentof the earth's magnetic field, means for producing a second electricalsignal varying with the dip of the earth's magnetic field, means forproducing a third electrical signal varying with the strength of theearth's magnetic field, all of said signal-producing means being carriedby said framework, and means for recording the variations of each ofsaid signals in correlation with depth in a well.

14. Apparatus for logging wells comprising an instrument housing adaptedfor lowering into a well, a reference framework suspended in saidhousing and rotatable relative thereto, means for maintaining saidframework oriented in a fixed direction, a plurality of coplanarsaturable ferromagnetic cores mounted in said framework and defining aplane of investigation of a magneticfield component, said cores being atleast partially saturable by flux from the well strata, means on eachcore for inducing periodically an additional saturating flux therein, apickup-coil means on said cores, connected in opposition in pairswhereby voltages from said additional saturating fluxes tend to cancelout leaving residual voltages varying with the well strata fiux in saidcores, means for combining said residual voltages to produce a resultantfield indicative of the strength and direction of said magnetic-fieldcomponent, and means for recording variations in the strength and thedirection of said component in correlation with depth in a well.

15. Apparatus for logging wells comprising an instrument housing adaptedfor lowering into a well, a reference framework suspended in saidhousing and rotatable relative thereto, means for maintaining saidframework oriented in a fixed direction, two sets of saturableferromagnetic cores mounted in said framework, each oi' said setscomprising a plurality of coplanar elongated cores defining a plane ofinvestigation of a mag-. netic-field component, one of said sets beinghorizontal and the other in a vertical plane including themagnetic-field vector at the earth's surface, and each of said coresbeing at least partially saturable by flux induced therein from the wellstrata, means for inducing in each core an additional periodicsaturating fiux, pickup-coil means on said cores responsive todifferences in said saturating fiux in said coresdue to the well stratafiux therein, meansfor deriving from said pickup means resultant fieldsindicating the direction of the magnetic-field component in the plane ofeach set of cores, means for measuring the total magnetic-field strengthin said well strata, and means for recording said total field strengthand the variations in direction of said a fixed direction,

' passing with depth in said well.

16. Apparatus for logging wells comprising an instrument housing adaptedfor lowering into a well, a reference framework suspended in saidhousing and rotatable relative thereto, means for maintaining saidframework oriented in a fixed direction, rotary means carried by saidframework and adapted for rotation about the longitudinal axis of saidframework, means on said rotary means for inducing an additionalmagnetic flux in well strata, pickup means on said rotary means adaptedto intercept a portion of the flux to the well strata from saidflux-inducing means, means for detecting an output from said pickupmeans having a frequency twice the frequency of rotation of the rotarymeans, and means for recording the phase of said output in correlationwith depth in a well.

1'7. Apparatus for logging wells comprising an instrument housingadapted to be lowered into a well, rotary means in said housingrotatable about the longitudinal axis thereof, means on said rotarymeans for inducing an additional magnetic flux in the strata of a well,pickup means on said rotary means adapted to intercept a portion of theflux passing to the well strata from said fluxinducing means, means fordetecting an output component from said pickup means having a frequencytwice the rotational frequency of the rotary means, an inductor coil forproducing an altemating voltage by rotation in the earths magneticfield, and means for recording the phase angle between said pickupoutput and said alternating voltage in correlation with depth in a well.

WILLIAM L. RUSSELL.

REFERENCES CITED The following references are of record in the file orthis patent: v UNITED STATES PATENTS Number Name Date 1,928,970 JohnsonOct. 3,1933 1,980,100 Schlumberger Nov. 6, 1934 2,220,788 Lohman Nov. 5,1940 2,262,419 'Athy Nov. 11, 1941 2,288,876 Arnold July 7, 19422,291,692 Cloud Aug. 4, 1942 2,401,280 Walstrom May 28, 1946 2,435,276

Holmes Feb. 3, 1948

